1. Field of the Invention
The present invention relates to a camera that performs automatic focusing by a phase difference detection method.
2. Description of the Prior Art
Some auto-focus (AF) cameras that automatically focus the taking lens on an object employs the phase difference detection method as the method of detecting the focus condition of the taking lens with respect to an object. In the phase difference detection method, a pair or a plurality of pairs of image detecting sensors such as line sensors or area sensors are arranged on a plane substantially equivalent to a predetermined focal plane where a film and an image sensing device such as a photoelectrically converting device are disposed, and luminous fluxes passing through different areas of the taking lens are directed to different sensors and the focus condition of the taking lens is detected from the distance between the object images on the sensors constituting a pair.
The positions of the object images on the sensors are determined based on the contrast of the images. Therefore, when the contrast of the object is insufficient, the positions of the object images on the sensors cannot be determined, so that the focus condition of the taking lens cannot be detected. Even when the contrast of the object is sufficient, in a case where the focus of the taking lens largely deviates from the in-focus position, the images blur on the sensors, so that the contrast is gone. Consequently, it is difficult to detect the focus condition.
When the focus condition cannot be detected because of insufficient contrast of the images on the sensors, the focusing lens for adjusting the focus included in the taking lens is moved at a comparatively low speed while monitoring the contrast of the images on the sensors, thereby finding the position of the focusing lens that provides the contrast where the detection of the focus condition is possible. This operation is called a low contrast scan or a low contrast search.
The low contrast scan is performed for the entire range where the focusing lens can be driven because it is uncertain whether the taking lens is focused on a position in front of the object or on a position in the rear of the object and how much the focus of the taking lens deviates from the in-focus position. For example, the focusing lens is moved, first, from the position at that time to the near end corresponding to the minimum focusable object distance and then, to the infinity end corresponding to the maximum (infinity) focusable object distance. By doing this, the focus condition can be detected even when the focus of the taking lens largely deviates from the in-focus position.
However, in conventional auto-focus cameras employing the phase difference detection method, the following problems arise when a rather special taking lens such as the one capable of macro photographing in addition to normal photographing is used in combination with the camera:
In macro photographing for photographing an extremely close object so as to be enlarged, since light from a minute area of the object is directed to the entire area of the sensors, the contrast of the images on the sensors decreases. Consequently, even a focus deviation that does not result in insufficient contrast in normal photographing results in insufficient contrast, so that the low contrast scan is performed.
Since a taking lens capable of macro photographing has a wide focusable distance range, the object is apt to be situated outside the range where the focus condition detection is possible. When the object is outside the range where the focus condition detection is possible, the images on the sensors blur, so that the contrast largely decreases. Consequently, the necessity for performing the low contrast scan increases.
In the low contrast scan, since the contrast of the images on the sensors is monitored while the focusing lens is being moved, it is inevitable that the contrast is decreased by the images flowing even though the focusing lens is moved at a comparative low speed. In the macro photographing, since the contrast is lower as mentioned above, it is necessary to perform the scan at a particularly low speed. However, if the scan is performed at a low speed, it will take a long time to find the position of the focusing lens where the focus condition detection is possible. Since the driving range of the focusing lens of the taking lens capable of macro photographing is wide, it takes an extremely long time to scan the entire range. This discomforts the user. This also halves the utility of the automatic focusing that requires swiftness.
In a bright taking lens having a particularly large aperture, since the focusing lens is large and heavy, a great force is necessary for driving the focusing lens. In such a case, if the entire range is scanned in the low contrast scan, much power will be consumed, so that the battery serving as the power source will become exhausted extremely soon. This will make it necessary to provide a large-capacity battery or to frequently change or charge the battery. As a result, the camera will increase in size and weight and inconvenience will be caused when the camera is used.